The callosal connections of the cat's auditory cortex will be examined anatomically using combined injections of H to the third power proline and peroxidase into one hemisphere. The orthograde transport of labeled proteins will demonstrate the location of callosal terminals in autoradiographs prepared from fixed sections of the brain. Retrograde transport of peroxidase will reveal the cell bodies that give rise to callosal axons in the same sections. The results will show the extent to which reciprocity exists in this set of connections. This question is of considerable interest, since the neurons of layer III that establish callosal connections are known to exist in discrete "patches" separated by gaps free of callosal cell bodies. These experiments will show if the gaps are also free of callosal terminals. Double labeling experiments utilizing DAPI-primuline and peroxidase will be made to examine the projections of individual cortical layers. Previous work in this project has shown that layer VI for example, gives rise to both cortico-thalamic and callosal connections. It is not known, however, if individual neurons in layer VI give rise to both types of connections. To resolve this issue, separate injections of DAPI-primuline (a fluorescent probe) and peroxidase will be made into tonotopically homonymous parts of the medial geniculate body and the contralateral auditory cortex. Sections of the primary auditory cortex will then be examined to see if individual cells contain both the fluorescent probe and the peroxidase reaction product, and hence send axons to both injected sites. Finally, the newly developed TMB procedure for demonstrating peroxidase activity reveals the dendritic trees of labeled neurons in some detail. The morphology of cells that give rise to cortico-thalamic, cortico-tectal and callosal connections will be correlated with the structure of neurons as seen in Golgi preparations to determine if distinct cell types within each layer give rise to these several pathways.